Preheat the oven to 350 degrees F. Lin the bottoms of two 8-inch round baking pans with parchment paper and spray with nonstick cooking spray. In a medium bowl, whisk together the egg whites (save the yolks for the pastry cream!), egg, buttermilk, vanilla, and almond extract, then set aside.
In a stand mixer fitted with a paddle attachment, combine the flour, cornstarch, sugar, baking powder, and salt. Add the butter one piece at a time, while the mixer is running on low speed, waiting 10 seconds between each addition.
Once all the butter is added, pour in the buttermilk and egg mixture in 3 batches, mixing on medium-low speed between each addition for about 1 minute and scraping the sides of the bowl. The batter should be light and fluffy. Once all of the liquid has been added, scrape the bottom and sides of the bowl well and mix for another 20 to 30 seconds.
Divide the batter evenly between the pans, then bake for 30 to 35 minutes, until a cake tester or toothpick inserted in the middle of the cakes comes out clean with just a few crumbs attached. Cool for 10 minutes before inverting the cakes onto cooling racks and removing the parchment paper from the bottoms. Flip the cakes back over and cool completely on the wire racks.
Level the tops of the cakes, then slice in half horizontally and wrap each cake layer with plastic wrap and chilling in the freezer for at least an hour before frosting.